1. Field of the Invention
This invention relates to an image signal recording and reproducing system for recording an image signal on a recording medium and reproducing the image signal from the recording medium.
2. Description of the Related Art
Hitherto, there has previously been known a still video (to be abbreviated to "SV" hereinafter) system as an apparatus for recording and reproducing a still image signal. The SV system is a system for recording a conventional TV signal on a two-inch magnetic disk after it has been subjected to an FM modulation. However, a system of the type described above is only able to realize a degree of resolution similar to that obtained in the conventional TV system. Furthermore, in the SV system which handles still images, the image printed out by a printer sometimes becomes the final output. In this case, there arises a problem in that the image quality (in particular, the resolution) is unsatisfactory in comparison to that obtainable in a silver-halide photograph.
Recently, novel TV systems such as HDTV (High-Definition TV) have been developed. The HDTV system has about 1,000 scanning lines, the number of which is twice the number of the scanning lines provided for the conventional NTSC system. Furthermore, a signal band in the horizontal direction which corresponds to the 1000 scanning lines is provided for the HDTV system. Therefore, it has been desired that the SV system be so improved that it also has a still image recording and reproducing system which exhibits the image quality of about 1000.times.1000 pixels (in a square frame) obtainable in the HDTV.
Accordingly, the SV system has been arranged in such a manner that the recording format for the recording medium has been made to be a high band (wide band) recording. However, the image quality must be improved while achieving a certain degree of compatibility between the SV system and a conventional system.
Therefore, it might be considered feasible to employ a CHSV (Compatible High-Definition SV) system capable of highly improving image quality while maintaining compatibility with the conventional system.
The CHSV system will schematically be described.
The CHSV system is based on a technology so-called "an analog transmission of a sample value".
As shown in FIG. 1, the system for analog-transmitting the sample value is characterized by the transmission path characteristics (LPF characteristics) and the re-sampling. That is, an input sample value is again sampled after it has passed though an FM modulation system, an electromagnetic conversion system and an FM demodulation system, so that the sample value is restored.
The principle of the analog transmission of a sample value will be further described with reference to FIGS. 2(a) to 2(f), where a case in which a series of sample values having a period T, as shown in FIG. 2(a), is recorded and reproduced is considered. A transmission path, consisting of an FM modulation system, an FM demodulation system and an electromagnetic conversion system, has low-frequency transmitting characteristics, that is, low-pass filter (LPF) characteristics. FIG. 2(b) illustrates the output of the transmission path. Therefore, when the output of the transmission path is re-sampled by using the resampling pulse having the period T and the correct phase as shown in FIG. 2(c), an output signal shown in FIG. 2(d) can be obtained. That is, the input series of sample values can be correctly reproduced (transmitted). However, if the phase of the re-sampling pulse is, as shown in FIG. 2 (e), deviated, the series of sample values cannot be correctly reproduced (transmitted). As a result, "ringing" is undesirably generated as shown in FIG. 2(f). Therefore, it is necessary for the analog transmission of a sample value to be performed in the reproducing mode (on the receiving side) in such a manner that: (1) Re-sampling pulses having the correct frequency (period) which corresponds to the reproduced (received) sample value signal are generated. (2) Re-sampling pulses having the correct phase which corresponds to the reproduced (received) sample value signal are generated.
Another condition must be met in order to perfectly transmit the sample value signal, that is: (3) The transmission path consisting of the FM demodulation system, the FM demodulation system and the electromagnetic conversion system must have the linear phases, and the frequency characteristics thereof must be the symmetric roll-off characteristics centering the frequency f.sub.s /2 (=1/2T) where f.sub.s is the sampling frequency. That is, the transmission path must have the LPF characteristics as shown in FIG. 3.
As briefly described above, the sample value is analog-transmitted.
Next, a method of recording a luminance (Y) signal based on the CHSV method will be described.
FIG. 4 illustrates sample points of the signal Y to be recorded on a magnetic disk based on the CHSV method. As shown in FIG. 4, the sample points of the signal Y are distributed in an offset manner so as to be transmitted in a sub-sampling manner. Assuming that 650 (=1300/2) sample points are provided for a line and 500 (=1000/2) sample points are provided for a column, all of the sample points are recorded by using four tracks in such a manner that the sample values included in lines A.sub.1, A.sub.2, . . . (the rest is omitted) are recorded on one track of the magnetic disk and the sample values included in columns B.sub.1, B.sub.2, . . . (the rest is omitted) are recorded on another track. The recording of sample points on each of the tracks is performed in accordance with the SV format. FIG. 5 illustrates the allocation of the frequency of the recording signal whose action is arranged to be based on the SV format. As shown in FIG. 5, the baseband widths for signals Y and C become about 7 MHz or lower and about 1 MHz or lower respectively in the SV format.
Assuming that the number of the sample points of the signal Y included in each of the lines is 650, the 650 sample points are recorded during a horizontal effective picture period (53 .mu.sec or less) for an NTSC-TV signal. Therefore, the sampling frequency f.sub.s (see FIG. 3) at this time becomes about 12.2 MHz or less. Thus, the signal Y having the band as shown in FIG. 5 can be recorded.
FIGS. 6(a) and 6(b) illustrate two recording patterns on the magnetic disk performed on the basis of the CHSV method. FIG. 6(a) illustrates a recording pattern when a 2-channel (ch) head is used, while FIG. 6(b) illustrates a recording pattern when a 4ch head is used (however, the recording patterns respectively shown in FIGS. 6(a) and 6(b) can be obtained when the 4ch head is used).
In the case shown in FIG. 6(a), first, the sample values of the signal Y for line A.sub.i (symbol "i" represents a positive integer) and line B.sub.i are simultaneously recorded on the first and second tracks. Then, the 2ch head is shifted to the third and fourth tracks (the shift is not necessary in the case on the 4ch head) so as to simultaneously record the sample values of the signal Y for the lines D.sub.i and C.sub.i. In order to keep compatibility with the conventional SV format, the tracks, on which the sample values of the signal Y for the respective lines D.sub.i and C.sub.i are recorded, are inverted with respect to each other.
In a case where the 2ch simultaneous recording operation is performed, there usually arises a problem in terms of crosstalk of the recording signal caused by the head during the recording. However, the problem of this type can be overcome since the known horizontal synchronizing operation is performed between the two heads at the time of the simultaneous recording operation in accordance with the aforesaid recording method.
In a case where the 4ch head is used, a recording operation as shown in FIG. 6(b) may be performed. That is, the sample values of the signal Y for the lines A.sub.i and B.sub.i are first and simultaneously recorded on the first and third tracks. Then, the sample values of the signal Y for the lines C.sub.i and D.sub.i are simultaneously recorded on the second and fourth tracks.
As a result, in the case shown in FIG. 6(a), the frame reproduction according to the conventional SV format can be performed by using the second and third tracks. In the case shown in FIG. 6(b), the frame reproduction according to the conventional SV format can be performed by using the first and second tracks or the third and fourth tracks.
Thus, the signal Y is recorded in accordance with the CHSV method.
Next, a method of recording a color-difference (c) signal based on the CHSV method will be described.
FIGS. 7(a), 7(b) and 7(c) illustrate the relationship between the recording sample patterns of C.sub.R (=R-Y) signal and C.sub.B (=B-Y) signal. In the conventional SV format, the recording band for a color-difference signal in a line-sequential form is one sixth of the signal Y. Therefore, the sample pattern for each of the color-difference signals C.sub.R and C.sub.B to be recorded in accordance with the CHSV method becomes as shown in FIGS. 7(b) and 7(c). The lines of the signal Y to be recorded on the same track on the magnetic disk are designated by symbols A.sub.i, B.sub.i, C.sub.1 and D.sub.i in the right portion of each of FIGS. 7(b) and 7(c). The reason for the existence of the portions, in each of which the line for the signal Y does not coincide with the line for the signal C, lies in the necessity of keeping compatibility with the SV format.
FIG. 8 illustrates the positional relationship between the signals Y and C, where expression "First Step" denotes "2ch simultaneous recording performed at the first time", while expression "Second Step" denotes "2ch simultaneous recording performed at the second time". As described above, in First Step, the recording on the tracks 1 and 2 is performed, while the recording on the tracks 3 and 4 is performed in Second Step. In the case shown in FIG. 8, for example, Y(A.sub.i) (the signal Y consisting of a series of sample values for the signal Y on the line A.sub.i shown in FIG. 7 (a)) and C.sub.R (A.sub.i)/C.sub.B (B.sub.i) are recorded, aforesaid C.sub.R (A.sub.i)/C.sub.B (B.sub.i) being a color-difference line-sequential signal consisting of the signal C.sub.R formed by a series of sample values for the signal C.sub.R on the line A.sub.i shown in FIGS. 7(b) and 7(c) and the signal C.sub.B formed by series of sample values on the line B.sub.i, the color-difference line-sequential signal being started with the C.sub.R signal. Referring to FIG. 8, outputs (Y.sub.1, Y.sub.2, R and B) of an imaging portion are signals which are simultaneously output from the image portion of a CHSV camera to be described later.
Next, the structure of the CHSV camera (an apparatus consisting of the imaging portion and a recording portion) will be described.
FIG. 9 schematically illustrates the structure of the CHSV camera.
As described above, the CHSV camera shown in FIG. 9 records image recording signals for one frame by successively performing the 2ch simultaneous recording twice. In First Step shown in FIG. 8, SV recording process circuits 826 and 827 respectively subject the supplied signals Y and C to a predetermined emphasis and an FM modulation so as to output signals formed by frequency-multiplexing the aforesaid signals. In adders 828 and 829, a pilot signal {the frequency of which is about 2.5 MHz (as is apparent from FIG. 5, the frequency of 2.5 MHz is positioned in the space between FM-Y and FM-C)} obtainable by passing a clock signal through a band-pass filter (BPF) 825 is added to output signals of the SV recording process circuits 826 and 827, the clock signal being generated by a clock generating portion 813 and the pilot signal serving as a reference signal for TBC (time base correction) in the recording mode. Signals output from the adders 828 and 829 are amplified by recording amplifiers 830 and 831 so as to be simultaneously 2ch-recorded on predetermined tracks of the magnetic disk 834 by 2ch heads 832 and 833. In Second Step, the recording operation similar to that performed in the aforesaid First Step is performed after the 2ch heads 832 and 833 have been shifted.
Next, an imaging portion 801 shown in FIG. 9 will be described.
FIG. 10 illustrates the structure of a color filter for use in a solid-state image sensor in a case where the imaging portion 801 comprises one solid-state image sensor. As shown in FIG. 10, the aforesaid color filter is constituted by luminance filters Y disposed in a checkered manner and filters R and B disposed in a line-sequential manner for the residual portions.
FIG. 11 illustrates an example of the structure of the imaging portion including the solid-state image sensor having the color filter constituted as shown in FIG. 10.
Referring to FIG. 11, reference numeral 1301 represents a solid-state image sensor having the color filter shown in FIG. 10, and 1302 to 1305 represent sample-and-hold circuits. In this case, the solid-state image sensor 1301 has about 1300 (horizontal pixels).times.1000 (vertical pixels) pixels and is capable of simultaneously reading, two lines apart, signals for two lines which are positioned vertically adjacent to each other.
Referring to FIG. 11, the signal Y (Y.sub.1) on the upper line of the signals for the two lines to be simultaneously read is transmitted to signal line (0-1). The signal Y (Y.sub.2) for lower line is transmitted to signal line (0-3), the signal R is transmitted to signal line (0-2) and the signal B is transmitted to signal line (0-4).
The sample-and-hold circuits 1302 to 1305 output the aforesaid signals after sample-holding them at predetermined timing.
FIG. 12 illustrates a schematic example of the structure of a case where the solid-state image sensor, which is capable of simultaneously, two lines apart, reading signals for two lines which are positioned vertically adjacent to each other, is constituted by MOS type solid-state image sensors.
The MOS type solid-state image sensor shown in FIG. 12 is an ordinary sensor arranged to act in accordance with a TSL (Transversal Signal Line) method.
Since the MOS type solid-state image sensor is arranged to read signals in accordance with an X-Y address method, the aforesaid two line simultaneous reading can be performed. A detailed description about the reading operation is omitted here.
Next, referring to FIG. 9, the process of inputting signals Y.sub.1, Y.sub.2, R and B to the SV recording process circuits 826 and 827 will be described for each of the signals Y and C, the signals Y.sub.1, Y.sub.2, R and B being produced by the imaging portion 801 which is driven by an imaging portion drive circuit 808 in synchronization with a synchronizing signal output from the clock generating portion 813.
As for the signal Y, the aforesaid signals Y.sub.1 and Y.sub.2 (see FIG. 8) output from the imaging portion 801 pass through LPFs 802 and 805, each of which has a passing frequency band of about 6 MHz, and gamma correction (.gamma..sub.Y) circuits 821 and 823 so as to be supplied to the SV recording process circuits 826 and 827.
The .gamma..sub.Y circuits 821 and 823 are transmission path gamma correction circuits provided for the purpose of improving signal-to-noise ratio in dark portions of a luminance signal and keeping compatibility with the conventional SV format.
As for the signal C, the aforesaid signals R and B (see FIG. 8) output from the imaging portion 801 pass through the LPFs 804 and 807, each of which has a passing transmission frequency band of 1 MHz, so as to be supplied to switch circuits S.sub.1 and S.sub.2. The switch circuits S.sub.1 and S.sub.2 are arranged to be switched on/off every 1H so that color line-sequential signals R/B (the output of the switch circuit S.sub.1) and B/R (the output of the switch circuit S.sub.2) are obtained.
In subtracters 809 and 810, the signal Y.sub.1 output from the LPF 803 having the passing frequency band of 1 MHz and the signal Y.sub.2 output form the LPF 806 having the passing frequency band of 1 MHz are subtracted from the outputs of the switch circuits S.sub.1 and S.sub.2, respectively. As a result, the color-difference line-sequential signal C.sub.R /C.sub.B is output from the subtracter 809 and the color-difference line-sequential signal C.sub.B /C.sub.R is output from the subtracter 810.
Then, the aforesaid signals are sampled by sample-and- hold circuits 811 and 812 so as to form the sample patters for the signals C.sub.R and C.sub.B shown in FIGS. 7(b) and 7(c). The sampling clock used at this time is generated by the clock generating portion 813.
Signals output from the sample-and-hold circuits 811 and 812 pass through LPFs 819 and 820 and gamma correction (.gamma..sub.c) circuits 822 and 824 so as to be supplied to the SV recording process circuits 826 and 827.
Next, the reproducing method will be described. FIG. 13 illustrates the schematic structure of a reproducing apparatus arranged to act in accordance with the CHSV method. A signal, reproduced from a magnetic disk 1501 serving as a recording medium by a magnetic head 1502, is amplified by a reproducing amplifier 1503 so as to be supplied to an SV reproducing process circuit 1504. In the SV reproducing process circuit 1504, the supplied signal is subjected to a separation process in which the luminance signal Y and the color-difference line-sequential signal C are separated from each other, subjected to an FM demodulation and to a deemphasis so that reproduced signals Y and C are output. The aforesaid two signals are gamma-inverse converted by gamma-inverse converters 1506 and 1507, and then the bands of the signals are restricted by the LPFs 1508 and 1509, respectively. Then, the signals pass through analog-to-digital (A/D) converters 1513 and 1514, respectively, so as to be supplied to an image memory 1515. The sampling clocks for use in the A/D converters 1513 and 1514 are formed by, first, extracting a pilot signal, which has been previously frequency-multiplexed in the recording mode, from a signal reproduced from the magnetic disk 1501, the extracting being performed by a band-pass filter (BPF) 1505. Then, the pilot signal and a synchronizing signal which has been separated by a sync separation circuit 1510 are used so as to form the sampling clock by sampling-clock generating circuits 1511 and 1512.
The aforesaid process is applied to the four tracks on the magnetic disk 1501 so that sample data output from the A/D converters 1513 and 1514 is stored on the address on the image memory 1515 designated by the address generators 1517 and 1518. The image processing circuit 1515 performs an interpolation by using the sample data stored in the image memory 1515. Then, the signals are read out from the image memory 1515 in a state in which the high-frequency component signal (Y.sub.H), the low-frequency component signal (Y.sub.L) and color-difference signals (C.sub.R and C.sub.B) are separated from one another. As a result, signals Y.sub.L, C.sub.R and C.sub.B are, as illustrated, supplied to a matrix circuit 1519 so as to be converted into three primary color signals (R.sub.L, G.sub.L and B.sub.L). Then, the high-frequency component (Y.sub.H) of the signal Y is added to each of the three primary color signals (R.sub.L, G.sub.L and B.sub.L) by adders 1520, 1521 and 1522 so as to be converted into analog signals by digital-to-analog (D/A) converters 1523, 1524 and 1525. As a result, the analog signals are output in the form of an RGB signal.
In the case of the CHSV method in which the sample value is analog-transmitted as described above, it is necessary to generate a correct re-sampling clock. Therefore, a sampling clock the phase of which is synchronized with the pilot signal reproduced from the magnetic disk is formed so as to be used for re-sampling the image signal reproduced from the magnetic disk and FM-demodulated. Furthermore, the image signal is stored in the memory in accordance with the aforesaid re-sampling clock so as to absorb jitters. In addition, when the image signal which has been re-sampled by the re-sampling clock is stored in the memory, the horizontal address for the image signal storing memory is reset in synchronization with a horizontal synchronizing signal and a vertical synchronizing signal added to the FM-demodulated image signal.
In a case where the aforesaid structure is employed, if the synchronizing signal portion of the demodulated image signal generates distortion of its waveform as shown in FIG. 14(a) or if the SN ratio is unsatisfactory, the disorder of the time base of the edge portion of the waveform cannot be prevented by only shaping the waveform, as shown in FIG. 14(b). As a result, when the horizontal address of the image signal storing memory is reset at the timing of the edge portion of the horizontal signal, the image signal may be stored in an address deviated from a normal address of the memory. Therefore, if the image signal which has been stored in the wrong address of the memory is read by using a correct reading clock signal, the reproduced signal would inevitably generate distortion. As a result, the original image cannot be accurately restored.
When the magnetic disk subjected to the recording by the CHSV camera is subjected to the reproduction by a reproducing apparatus structured in accordance with the CHSV method, there arise the following problems:
When the magnetic disk subjected to the recording by the CHSV camera is subjected to the reproduction by a reproducing apparatus structured in accordance with the CHSV method, the "correct re-sampling phase" which is one of the aforesaid conditions to realize the analog transmission of a sample value cannot be achieved only by the pilot signal recorded on the magnetic disk in the frequency multiplexed manner.
That is, the CHSV camera is arranged in such a manner that the processing system for the image signal and the system for the pilot signal are composed of different circuits and filters. Therefore, the relationship in terms of time delicately differ between the image signal and the pilot signal depending upon the kind of the CHSV camera which performs the recording.
Therefore, when the magnetic disk subjected to the recording by a predetermined CHSV camera is subjected to the reproduction, the adjustment of the re-sampling phase in the reproducing apparatus by forming the re-sampling clock signal the phase of which has been synchronized with the TBC pilot signal, must be performed again if a magnetic disk subjected to the recording by another CHSV camera is subjected to the reproduction. As a result, if the point, which is re-sampled by the re-sample clock signal in the reproducing mode, deviates from the sample point in the recording mode, the aforesaid condition for analog-transmitting the sample value cannot be satisfied. Therefore, the quality of the reproduced image deteriorates. Accordingly, there arises a desire of a structure capable of automatically adjusting the re-sample phase. However, there has not been reference information acting to automatically adjust the re-sample phase for use in the conventional CHSV camera.